1. Field of the Invention
The present invention relates to a biological information detecting device for extracting biological information from an image detected by scanning biological information having an undulating structure composed of peak parts which protrude at least from its surroundings and trough parts which are lower than their surroundings.
2. Description of the Related Art
Conventionally, when outputting a biological information image to a personnel authentication device for identifying the relevant person, a skin status analyzing device for analyzing the status of a biological epidermis or the like in a biological information extraction device, the value of pixel of a biological information image varies depending on variations of factors, such as variations of individuals, the environment, machine variations and the like. Therefore, it is necessary to eliminate such variations factors as much as possible and to extract stable biological information. In a fingerprint image or a skin image in particular, if the epidermis is dry, the contrast of the value of pixel of biological information is low. Therefore, it is necessary to prevent part of the biological information from becoming blurred or being lost. If the epidermis is wet, the contrast of the value of pixel is high. Therefore, it is necessary to prevent biological information from being ruined by sweat and the like.
As technologies for improving the contrast of the value of pixel, the following are publicly known.
In patent reference 1 the pixel values of a fingerprint image integrated and it is determined whether to output the fingerprint image, using the peak value of a value of pixel level. Since a fingerprint must be sampled several times, sometimes a finger, which is a subject, moves during the sampling. In such a case, no appropriate biological information can be obtained, which is a problem. In the case of a dry finger, the number of samples increases, which is a problem especially.
As technologies for not integrating the pixel values, there are Patent references 2 and 3. In these technologies, the respective effects of dry/wet fingers are nullified by equalizing the histogram of a fingerprint image or expanding its value of pixel distribution using a general contrast control method. In these technologies, if there is sensitivity variation or noise, peculiar to a sensor in a fingerprint, the contrast of the noise as well as that of the biological information increases. Therefore, if the value of pixel of the noise is greater than that of a dry finger, it is difficult to collect an appropriate fingerprint image.
As technologies for eliminating noise, such as the sensitivity variations of a sensor, there are Patent references 5 and 6, in which value of pixel correction generally used in an image scanner as disclosed by Patent reference 4 is applied to a fingerprint image. In these technologies, a background image is initially obtained and then is subtracted from the sampled image. However, in Patent references 5 and 6, a difference is calculated simply, and does not correspond to the image sampled after adjusting the sensitivity in order to collect an appropriate dry/wet fingerprint. Furthermore, if there are sensitivity fluctuations in a sensor as well, it cannot be used. As reference literature that addresses the problem of the sensitivity fluctuations of a sensor, there is a Non-patent reference 1.
There are also technologies for outputting whether or not biological information is contained. In the above-mentioned Patent reference 1, the value of pixel values of a fingerprint image are integrated and it is determined whether to output biological information, using the peak value of the value of pixel level. Patent reference 7 analyzes a histogram for each block and counts blocks containing a ridge, based on a predetermined threshold value. Patent reference 8 blurs an input image using a low-pass filter, and uses it as a mask pattern for separating the binarized image into the background and biological information. In any of the technologies, if a residual fingerprint adheres to a contact type sensor there is a possibility that patterns other than the desired biological information, such as sensitivity fluctuations, a residual fingerprint and the like may be output as meaningful information.
Generally, the size of a sensor is larger than the extraction range of the biological information. Therefore, the size of a sensor contains a lot of redundant information. However, in any of the above-mentioned technologies, the size of a sensor determines neither the location nor range of biological information.
Patent Reference 1:
Japanese Patent Application Publication No. 2001-319234
Patent Reference 2:
Japanese Patent Application Publication No. H8-110860
Patent Reference 3:
Japanese Patent Application Publication No. 2000-322559
Patent Reference 4:
Japanese Patent Application Publication No. H2-203671
Patent Reference 5:
Japanese Patent Application Publication No. H11-134498
Patent Reference 6:
Japanese Patent Application Publication No. 2001-521665
Patent Reference 7:
Japanese Patent Application Publication No. 2002-298126
Patent Reference 8:
Japanese Patent Application Publication No. 2003-44856
Non-Patent Reference 1:
NEC Technical Report Vol. 55, No. 3/2002, p. 21, FIG. 4, Upper left